The present invention relates to an apparatus and method for automatically observing or classifying defects caused on a semiconductor wafer in a process for producing a semiconductor product.
In a process for producing a semiconductor product, in order to ensure high producing yield, it is necessary to be found various defects caused in the production process and to be taken measures for generation of the various defects, at an early stage. This is typically performed in the following steps.
(1) Detecting the locations of the caused defects or the deposited foreign particles by that a semiconductor wafer to be inspected is inspected by a wafer visual inspection unit or a wafer foreign particle unit.
(2) Classifying the detected defects under each of generating causes by observing (this is called a review) them. This review operation typically uses a review-specific unit having a microscope for observing defect parts at high magnification. Other unit with a review function, e.g., a visual inspection unit may be also used.
(3) Solving measures for each of the causes are taken.
In the case that the number of defects detected by the inspection unit is very high, the review operation needs enormous efforts. Therefore, in recent years, there has been developed actively a review unit having an Automatic Defect Review function for automatically imaging and collecting images of defect parts and an Automatic Defect Classification function for automatically classifying the collected images. Japanese Opened Patent Publication No. Hei 10-135288 discloses a review unit having an Automatic Defect Review function and an Automatic Defect Classification function and using a scanning microscope for an imaging system of it, and a production system.
FIG. 2 shows one example of a prior art ADR processing flow. First, a wafer to be inspected is placed onto the stage of a review unit (S21) and inspection data as results inspected by the inspection unit is read into the review unit from the database (S22). Then, the operator selects and specifies the defect targeted for ADR from the inspection results obtained from the inspection unit (S23). When the throughput of the ADR is high and the defected defect data are small in number, all the defects can be subject to ADR.
The review unit selects one from the specified defects and moves the stage so that the selected defect is positioned in the center of the visual field of an observation system. Thereafter, optimal focus setting is performed and an image of the selected defect is imaged by the observation system (S24). This image is called a defect image. The imaged defect image is stored into a recording medium (e.g., a magnetic disk) in the review unit.
Next, while the stage is moved, an image of the same part of the chip adjacent to the semiconductor chip in which the defect part in the wafer exists, is imaged (S25). This part is formed with the same pattern as that of the defect part. This image is called a reference image to the defect image. The reference image is also stored into the recording medium in the review unit. At the completion of imaging of the reference image, the defect image and the reference image of the next defect are imaged, as described above. These processes are repeated for all the defects to be subject to ADR, and are then terminated (S26).
FIG. 3 shows one example of a prior art ADC processing flow. ADC is a process for automatically deciding a category of the defect by using the defect image and the reference image acquired by ADR. First, the defect part is specified from the defect image and the reference image (S31). Specifically, a differential image is generated by being differentially operated between the defect image and the reference image. As a result, only the part in which the defect image and the reference image are different from each other appears in the differential image, which exhibits the defect part. The feature amounts of the defect are calculated by using the differential image, the defect image and the reference image (S32). The feature amounts quantitatively and numerically express the size of the defect, the shape of the defect, and the contrast on the image of the defect. An Automatic Defect Classification process for deciding the defect category is performed by using the feature amount data (S33).
The prior art ADR and ADC shown in FIGS. 2 and 3 are disclosed in Japanese Opened Patent Publication No. Hei 10-135288. In the prior art, the defect image or the reference image is imaged after the stage is stopped once. The imaging of one imaged part consists of three steps for: (1) moving of the stage to the imaged part, (2) stopping of the stage, and (3) imaging of an image.
When the stage is stopped for imaging, stage control and beam control during imaging can be simplified. On the other hand, the stage must be stopped completely. As the stage has some weight, if a stop command is issued from the control unit of the stage, the stage will not be stopped soon and the time to stop the stage completely is required to some extent. As the stage is moved slightly while the time to stop the stage elapses, if a review image is imaged when the stage is moved slightly, blurring or flow is caused in the image. As the result, the image quality needed for review cannot be obtained. For this reason, it must be waited to start imaging the review image until the time to stop the stage elapses after the command to stop the stage is issued. As this time is longer than the time needed for imaging, the prior art imaging method cannot acquire an image fast.
Here, consider the limit of throughput on the prior art. For simplification, chips produced in a wafer have a 15 mm pitch, and the number of defects per chip is 1. In other words, assume that the interval between defects and the interval between the defect part and the reference part are about 15 mm. The stage moving velocity is assumed to be 50 [mm/sec]. In this case, the time to move a distance of 15 mm is 15/50=0.3 [sec]. Actually, the stage moving needs acceleration or deceleration, and this time must be considered. However, this time is omitted here. The waiting time from stopping of the stage to starting of imaging is 0.2 [sec] as an experience value.
For imaging, a beam needs scanning in two dimensions. If an image of 512xc3x97512 pixels as one frame is acquired by scanning at 100 MHz, that is, 10 [n sec/pixel] for one pixel, it need 512xc3x97512xc3x9710 n[sec]=about 3 [m sec]. As a scanning electron microscope causes much noise in a detected signal, frame addition is typically performed to acquire a high-quality image. The number of the frame additions is assumed to be 16. In the operation of frame addition, a plurality of images of the same part are imaged, so that an average gray-scale value of the same pixel over the plurality of images is obtained as a pixel value of the same pixel, thereby acquiring an image reducing the influence of the noise.
As the number of frames is increased, the image quality is enhanced, but long time is required accordingly. The number of frames is set by considering image quality to be acquired. When 16 frame additions are performed, an electric current value of an irradiation beam is, for example, 200 [pA]. A signal amount n for irradiation to one pixel is n=(200 [pA]xc3x9710 [nsec]xc3x9716)/1.6xc3x9710xe2x88x9219=200 (one electron is 1.6xc3x9710xe2x88x9219 [coulomb]. Since a noise of the signal n by statistical fluctuation is xcex94n=nxc2xd, in this case, xcex94n=14.1. As an index to quantify the image quality, a standard deviation "sgr" of noise variation to a signal is used to assume that the fluctuation amount xcex94n of the signal is 3"sgr". From 3"sgr"=14.1, "sgr"=4.7 is determined. With this value, it is found from experience that the image quality needed for review can be ensured.
In imaging, other than the beam scanning, AF (Auto Focus) must be controlled. The time required to analyze various control commands by the internal computer is also needed. These times are different due to the control method or system. Here, the time is 0.5 [sec] as an experience value. As a result, about 3 [msec]xc3x9716+0.5=0.55 [sec] is calculated, which is required to image images consisting of 16 frames.
Imaging for one defect needs the steps for: (1) moving to the defect part (moving the stage by 15 mm), (2) waiting for stage stopping, (3) imaging, (4) moving to the reference part (15 mm), (5) waiting for stage stopping, and (6) imaging. The time needed for this is calculated using the above-mentioned value to determine about 0.15+0.2+0.55+0.15+0.2+0.55=1.8 [sec]. It is converted to the number of defects imaged per hour, a throughput is 3600 [sec]/1.8=2000 DPH (Defect Per Hour). An upper limit of the throughput of the prior art ADR can be considered about 2000 DPH. The throughput of a review unit using a scanning electron microscope currently on the market is several hundred DPH. With the influence of various overheads caused in each of the units, it is found that the throughput lower than the upper limit test-calculated above can be only realized.
Making semiconductor products finer is advanced increasingly. The number of defects detected from one wafer is enormous. It is sufficiently possible that about 10000 defects can be detected from one wafer. The throughput of the visual inspection unit or the foreign particle inspection unit for detecting these defects is increased. By way of example, the throughput of a foreign particle inspection unit using a laser scattering light detection method is about 100 [sec] per wafer. The throughput of the foreign particle inspection unit is thus high. The foreign particle inspection unit is often used in the production process as a tool for inspecting the same wafer to examine the transition of the number of defects.
To grasp what defect is caused in a wafer, all the defects on the wafer must be reviewed (100% review). In the prior art, the 100% review requires above five hours per wafer. In the production process, the time for inspection and review does not contribute directly to production. It is not preferable that one wafer is reviewed for above five hours. For this reason, several to several hundred defects are sampled from all the defects. Only the sampled defects have been reviewed.
In sampling, when the defect type is unbalanced, the defect causing state cannot be grasped suitably. As the result, it can be impossible to make full use of the inspection result data.
Therefore, there will be required in the future a unit capable of reviewing the defect data of all the defects data (about 10000) on one wafer for about one hour.
In the prior art, when images of a plurality of defect parts detected by the defect inspection unit are imaged automatically, imaging is performed after the stage is positioned for each of the defects so that the defect part comes into the center of the visual field of the imaging system such as a microscope. In other words, imaging is performed after the stage is stopped once for each of the defects in the position in which the defect comes into the center of the visual field.
Since the stage has some weight, several seconds is typically needed from issuing of a stop command from the stage control system to actual stopping of the stage. During the several seconds, the stage is moved slightly. When imaging is performed in this state, blurring or flow is caused in the image. An image capable of corresponding to the review cannot be acquired. When the stage is stopped for imaging, the imaging must be waited while the stage is stopped completely.
In the prior art, the stage is moved or stopped for each of the imaged parts. The throughput of the imaging cannot be increased.
The present invention solves the foregoing problems of the prior art, and an object of the present invention is to provide an image collection apparatus capable of collecting images of the observed parts with high throughput.
Accordingly, in the present invention, a scanning electron microscope is used as an imaging system to control the irradiation position of an electron beam based on the positional information of a stage and the positional information of an imaged part, whereby imaging is possible while moving the stage.
According to the present invention, in a method for moving a stage with a sample placed thereon based on the positional information of a plurality of locations to be observed on the sample so as to sequentially place the plurality of locations to be observed into an observation visual field which are observed sequentially, the location to be observed placed into the observation visual field is imaged while moving the stage to acquire an image of the location to be observed, the acquired image is classified, and the classified image is stored.
According to the present invention, in a method for moving a stage with a sample placed thereon to sequentially image a plurality of locations on the sample, and classifying images acquired by the imaging, the stage is moved based on the positional information of the plurality of locations to be imaged on the sample to sequentially place the plurality of locations to be imaged into an imaging visual field, the location to be imaged placed into the imaging visual field is imaged while moving the stage, and then, 10000 images can be acquired for one hour, whereby the acquired image is classified to be stored.
According to the present invention, in a method for moving a stage with a sample placed thereon to sequentially image a plurality of locations on the sample, and classifying images acquired by the imaging, the stage is moved at a first velocity based on the positional information of a plurality of locations to be imaged on the sample to sequentially place the plurality of locations to be imaged into an imaging visual field, the location to be imaged placed into the imaging visual field is imaged while moving the stage at a second velocity lower than the first velocity to acquire an image of the location to be imaged, the acquired image is stored.
According to the present invention, in a method for sequentially imaging a plurality of locations on a sample to classify the images acquired by the imaging, the stage with a sample placed there on is moved based on the positional information of the plurality of locations to be imaged on the sample to sequentially place the plurality of locations to be imaged into an imaging visual field, and the location to be imaged placed into the imaging visual field is imaged while moving the stage, whereby images of the plurality of locations to be imaged on the sample are acquired without stopping movement of the stage, and the acquired images are classified.
According to the present invention, in a method for observing a sample at a first magnification to detect a plurality of defects, storing the positional information of the plurality of defects detected, and observing the sample at s second magnification higher than the first magnification based on the positional information of the plurality of defects stored, whereby when the plurality of defects are observed at the second magnification, the plurality of defects can be observed without stopping movement of the sample.
According to the present invention, in a method for sequentially imaging a plurality of locations on a sample to acquire an image, the order of moving the sample is set so that the plurality of locations are sequentially placed into in an imaging visual field, the sample is moved to the imaging visual field in accordance with the set order, and the location placed into the imaging visual field of the plurality of locations is imaged while moving the sample to acquire an image.
These and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.